1. Field of the Invention
The invention relates to plunger type locks and particularly to locks which have an operating mechanism which is unlocked when a key is inserted axially into the lock to the proper depth and then partially withdrawn.
2. Description of the Prior Art
Plunger type locks are disclosed in U.S. Pat. Nos. 3,002,368; 3,714,802; 3,033,016; 3,478,548; 3,835,674; 4,015,456; 4,040,279; 4,058,992 and 4,155,232 and are relatively widely used to prevent tampering with gas, water and electric meters, unauthorized operation of valves and switches, etc. In general, such locks have a hollow barrel containing a plunger axially movable in the barrel from a locking position to an unlocking position, the plunger being spring biassed into the locking position. One end of the barrel is receivable in a cap or housing having a recess or groove. In the locking position of the plunger, a plurality of locking members, in the form of steel balls are held by the plunger, partly extending outwardly of the barrel and received in the recess of the cap or housing so that the barrel cannot not be removed from the cap or housing.
To unlock the lock of some prior art constructions, and thereby to permit separation of the barrel from the cap or housing, a key with spreadable fingers at its end is inserted into an opening in one end of the barrel and is moved lengthwise or axially of the barrel until the fingers are received within a socket at the end of the plunger, at which time, the fingers are spread apart causing them to engage the wall of the socket in the plunger. Thereafter, as the key is moved axially in a direction opposite to the key insertion direction, the plunger moves axially of the barrel permitting the steel balls to move inwardly of the barrel and out of the recess in the cap or housing.
In other types of plunger type locks, e.g. those disclosed in U.S. Pat. Nos. 3,478,548; 4,015,456 and 4,058,992, the operation is similar except for the use of a "grabber" type key to grab or grip a pin associated with the plunger rather than a key with spreadable fingers for engaging the wall of a socket in the plunger.
It is known in the lock art that persons seeking unauthorized access to utility meters, gas shut-off valves, current transformer cabinets, switches, etc., are resourceful and adopt ingenious ways to open the locks used. As a consequence, considerable effort has been devoted to the development of economical and sturdy locks for locking such devices which are resistant to opening thereof by unauthorized means. One type of expedient used by unauthorized persons is a nail, rod, tubing or improvised tool inserted into the lock in such a manner as to engage the wall of the plunger socket upon retraction of such nail, rod, tubing or tool in the same way that it is moved by a correct key. Another expedient is to obtain unauthorized access to an incorrect key and to manipulate such incorrect key in a way which will cause unlocking movement of the plunger.
One object of the invention is to provide a plunger type lock in which the plunger is locked in its locking position whenever an attempt is made to pick the lock with an implement which can be inserted into the lock farther than an authorized key can be inserted therein and/or whenever a key or implement not having the correct keying positions of an authorized key is used.
Another object of the invention is to make locks of the type which are operable by a "grabber" type key more resistant to picking by an improper key or an implement other than a key by locking the plunger in its locking position whenever an attempt is made to pick the lock with such a key or implement.